1. Field of the Invention
This invention relates generally to mooring systems for offshore terminals and in particular to offloading apparatus and methods for spread moored FPSOs (floating production storage and offloading vessels).
2. Description of the Prior Art
The spread mooring of FPSO vessels with offloading by tandem connection of a shuttle tanker is well-known in the prior art. Prior art tandem connection of a shuttle tanker to an FPSO for hydrocarbon offloading are characterized by several problems:                (1) The limited sector available to the shuttle tanker for unloading at the bow or stem of the vessel (centerline dead astern or dead ahead to ±30 degrees to port or starboard).        (2) The proximity between the shuttle tanker and the FPSO required for tandem offloading, during approach and offloading with the possibility of collision in severe weather.        (3) The FPSO's inability to weathervane.        (4) The magnitude of potential damage in the event of collision.        (5) The cost of maintaining the shuttle tanker within the safe unloading zone during offloading.        (6) The cost of assisting the shuttle tanker during approach to the FPSO.        
Summing up, prior offloading systems and methods for tandem offloading from a spread moored vessel to a shuttle tanker results in collision risk and unloading downtime risk.
To reduce risks, prior art systems are known which provide an SPM (Single point Mooring) terminal at a distance of 2000 meters from the FPSO for offloading. Such an arrangement permits weathervaning of the shuttle tanker, eliminates proximity to the FPSO, reduces the cost of collision between the shuttle tanker and the terminal, and minimizes the cost of shuttle tanker assistance.
Such prior art spread moored FPSO offloading systems and methods have provided an independent SPM for a shuttle tanker such as a CALM buoy located a long distance (usually about 2000 meters) from the FPSO in order that a shuttle tanker not contact the FPSO. A flow line, such as a steel pipeline, is run from the FPSO to the CALM buoy. A hose is then run, via a rotatable fluid coupling, to the shuttle tanker which is moored to the CALM buoy by means of a mooring hawser. Fatigue problems (due to constant movement of the sea surface) in the pipe line where it connects to the CALM buoy have been overcome by terminating the pipeline at a submerged Flowline Termination Buoy (FTB). A flexible hose is run from the pipeline end at the FTB to the CALM buoy.
Such prior art systems have provided complete independence of the SPM for the shuttle tanker due to the great distance between the tanker and the FPSO. In other words, the CALM buoy, to which the shuttle tanker is moored, is anchored to the sea floor without any mooring members connected to the FPSO. Unfortunately, in deep water, the cost of the mooring system, SPM terminal, and the fatigue resistant flow line from the FPSO to the FTB is very high and justified only for installations with high throughput and consequent high frequency of offloading with resulting higher risk.
Identification of Objects of the Invention.
The object of this invention is to provide arrangements and methods which overcome the disadvantages identified above.
Another object of the invention is to provide a single point mooring (SPM) for a shuttle tanker where the SPM is controlled directly or indirectly by linkage to a spread moored FPSO, with the result that the disadvantages identified above are overcome.
Another primary object of the invention is to provide a mooring system by which a shuttle tanker is moored to a SALM which is directly linked to a submerged yoke which is pendularly connected to the FPSO such that the shuttle (1) can be moored to an FPSO with the mooring being tolerant to surge conditions of the sea, (2) can accept connections of the shuttle tanker at angles to the longitudinal axis of the FPSO, and (3) can allow weathervaning angles of the shuttle tanker with respect to a spread moored FPSO up to about 300 degrees.
Another object of the invention is to provide a mooring system by which an LNG shuttle tanker is moored to buoyant columns secured to a submerged yoke which is pendularly connected to an LNG/FPSO such that the tanker is (1) surge tolerant, (2) can be moored at angles to the longitudinal axis of the LNG/FPSO and (3) can rotate in an arc about an end of the LNG/FPSO.
Another object of the invention is to provide a mooring system by which a shuttle tanker is moored at one end to a hold back buoy which is indirectly linked to the FPSO by means of a tension member connected between an end of the FPSO and an opposite end of the shuttle tanker, such that the shuttle tanker (1) can move in an arc about the end of the FPSO (2) is prevented from contacting the FPSO by the hold back buoy and (3) can be quickly disconnected from the hold back buoy.
Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy spaced about 600 meters from the end of the spread moored FPSO, where the mooring buoy is anchored to the sea floor and linked to the FPSO by means of a catenary chain, such that the shuttle tanker can move in a three hundred sixty degree circle about the mooring buoy without contact with the FPSO.
Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy in the form of a SALM which is connected to a mooring leg group for the FPSO, where the SALM is spaced about 600 meters from the end of the spread moored FPSO, such that the shuttle tanker can weathervane in a three hundred sixty degree circle about the SALM without contact with the FPSO.
Another object of the invention is to provide a mooring system by which a shuttle tanker is moored to a mooring buoy in the form of a Dynamically Positioned buoy, indirectly linked to the FPSO by means of a remote control link and directly linked to the FPSO by means of a mooring line between the DP buoy and the FPSO, where the DP buoy is spaced about 600 meters form the end of the spread moored FPSO, such that the shuttle tanker can weathervane in a three hundred sixty degree circle about the DP buoy and the DP buoy can be positioned in an arc about an end of the FPSO.